A digital image comprises an array of pixels. One or more values specify image characteristics for each pixel. Image characteristics may comprise, for example, luminance, chromaticity, luminance for individual color channels, and the like. Digital images may be represented in any of a wide array of formats. Non-limiting examples of image formats are RGB, XYZ, Luv, CIE LAB, and many others. One difference between different image formats is what image characteristics the pixel value(s) specified by the format represent.
Digital images may be adjusted for display by different devices and/or to provide a desired visual effect. Some non-limiting examples of image adjustments are increasing or decreasing dynamic range, increasing or decreasing contrast, increasing or decreasing average pixel values and the like.
A global adjustment comprises a mapping or transformation that may be applied to all pixels of an image. By contrast, a local adjustment applies different mappings or transformations to pixels within different local areas of an image. A global adjustment may be represented by a function that takes as input one or more pixel values for a pixel (the pixel values may comprise, for example values for the pixel in R-G- and B-channels) and provides as output one or more corresponding adjusted pixel values for the pixel.
It can be beneficial to adjust digital images for viewing on particular display apparatus. Display apparatus may employ any of a wide range of technologies. Some non-limiting examples are plasma displays, liquid crystal displays (LCDs), cathode ray tube (CRT) displays, organic light emitting diode (OLED) displays, projection displays that apply various light sources and various spatial light modulation technologies, and so on. Displays may be of a wide variety of types such as televisions, computer displays, displays in hand-held devices, displays used on control panels for equipment of different kinds, electronic game displays, digital cinema displays, special purpose displays such as virtual reality displays, advertising displays, stadium displays, medical imaging displays, and so on. Different displays may have different capabilities in areas such as: black level, maximum brightness, color gamut, and so on. A digital image may be optimized for viewing using a particular display apparatus by performing suitable image adjustments. As a simple example, an image in a high dynamic range (HDR) format may be adjusted for viewing on a display having a relatively low dynamic range.
Existing image-editing software such as, for example, Adobe™ Photoshop, Apple Aperture™ and others permit users to edit pixel values on a pixel-by pixel basis and to select and apply any of a wide range of transformations to pixel values. Professional editing suites and color grading stations also provide such capabilities. Such systems permit users to alter the appearance of images by setting parameters to define and applying any of a vast number of possible transformations. However, not all adjustments improve the appearance of images. Poorly chosen adjustments may obscure image details, introduce noticeable artifacts of various types, and/or alter the depictions of subjects in an image in non-natural ways. It takes significant skill to select transformations that will make any particular image have a desired visual effect. It can be time consuming for even a skilled image editor to use existing software tools to adjust a digital image to provide a desired visual effect.
There is a need for automatic or semi-automatic methods and apparatus for adjusting digital images for viewing.